The coefficient of kinetic (dynamic) friction quantifies the sliding resistance between two surfaces and is a decisive quality parameter for plastic films, paper, rubber, textiles, and floor coverings. This technical guide explains how accredited third-party laboratories determine the dynamic coefficient of friction, covering the measurement principle, key standards such as ASTM D1894, ISO 8295, GB/T 10006, and EN 13893, sample requirements, test procedures, calculations, and typical industrial applications.
Technical Overview
The coefficient of kinetic friction, commonly referred to as the dynamic friction coefficient, is defined as the ratio of the tangential force (Fk) required to maintain sliding between two surfaces at a constant relative speed to the normal force (N) pressing the surfaces together. It is a dimensionless quantity, expressed as the Greek letter mu (μk), and is calculated as μk = Fk / N.
Unlike the static coefficient of friction, which describes the force needed to initiate movement from rest, the kinetic coefficient characterizes resistance during sustained sliding. This is the condition that actually governs real-world behavior such as film travel over packaging machine rollers, paper feed through printing presses, carton movement on conveyor lines, and shoe-floor interaction during walking.
The dynamic coefficient of friction is one of the most widely specified surface properties in material quality control. A film that is too slippery cannot be tracked or stacked reliably, while a film with excessive friction jams converting equipment and slows production lines. For floor coverings, adequate dynamic friction is directly linked to pedestrian slip-resistance and safety compliance. The principal international standards governing its determination include ASTM D1894 and ASTM D4917, ISO 8295, GB/T 10006, and EN 13893, together with conditioning standards such as ISO 291 and ASTM D618.
Testing Samples
Plastic Films and Sheeting
Single-layer and laminated films of PE, PP, BOPP, PET, PA, PVC, and aluminum-containing composites used for bags, pouches, labels, and form-fill-seal packaging. Testing is typically performed film-to-film (outer surface to outer surface, or sealant side to sealant side) or film-to-metal to simulate contact with machine parts such as stainless steel guides and rollers.
Paper and Paperboard
Printing and writing papers, copy paper, release liners, folding carton board, and corrugated board. Friction testing evaluates feed reliability in printers, copiers, and cartoning lines, and is usually conducted separately in the machine direction and cross direction.
Rubber, Elastomers, and Foam Materials
Gaskets, seals, conveyor belting, shoe soles, glove surfaces, medical device components such as syringe plungers and stoppers, and soft-touch elastomeric coatings.
Textiles and Nonwovens
Hygiene nonwovens, interlinings, technical fabrics, and geotextiles, where fabric-to-fabric and fabric-to-metal friction influence processing, lamination, and end-use handling.
Floor Coverings and Coated Rigid Surfaces
Resilient flooring (vinyl, linoleum, rubber), laminate flooring, ceramic and coated tiles, and painted or lacquered surfaces, tested for slip resistance under dry and, where applicable, contaminated or wet conditions.
Testing Items
- Kinetic (dynamic) coefficient of friction, film-to-film, for plastic films and laminates
- Kinetic coefficient of friction, film-to-steel or film-to-glass, simulating packaging machine contact
- Static coefficient of friction as a paired determination on the same specimens
- Directional evaluation in machine direction and cross direction for films, paper, and board
- Dynamic coefficient of friction of floor coverings for slip-resistance assessment
- Coefficient of friction after heat aging, humidity exposure, low-temperature conditioning, or surface treatment
- Slip-agent migration and bloom studies by measuring friction changes over time or after storage
Testing Methods
Horizontal Plane (Sled-Type) Method
This is the dominant method for flexible materials and is specified in ASTM D1894, ISO 8295, and its Chinese equivalent GB/T 10006. A flat test plane carries a strip of one specimen, and a second specimen is wrapped around or bonded to the underside of a standardized sled. In ASTM D1894 the sled has a 63.5 mm square base and a mass of 200 g; ISO 8295 and GB/T 10006 use a comparable 200 g sled. The sled is connected through a flexible filament to a force-measuring load cell and is drawn across the plane at constant speed, typically 150 mm/min per ASTM D1894 and 100 mm/min per ISO 8295.
- Prepare plane and sled specimens without touching the test surfaces, marking machine direction where relevant.
- Place the sled on the plane, attach the filament to the load cell, and eliminate slack without applying preload.
- Start the drive and record the force-distance curve over a fixed sliding travel, commonly on the order of 70 to 150 mm.
- Read the initial force peak as the static friction force and the average force during steady sliding as the kinetic friction force.
- Repeat the test on fresh specimen areas, usually a minimum of three to five replicates per condition.
Inclined Plane (Ramp) Method
In the ramp technique, one specimen is fixed to a tilting platform and a sled or slider carrying the counter-surface rests upon it. The platform is raised slowly until sliding just begins, and the tangent of the angle at onset yields the static coefficient of friction. Although the ramp method fundamentally measures the initiation of motion, it is frequently used in slip-resistance screening and in routine comparative checks because of its simple apparatus. Dedicated ramp-based and slider-drag procedures are also applied to flooring to evaluate behavior during continued sliding.
Dynamic Friction Testing of Floor Coverings
For resilient, laminate, and textile floor coverings, EN 13893 specifies the determination of the dynamic coefficient of friction using a motorized sliding tester that drags a loaded slider across the surface at a defined speed. The mean sliding resistance is converted into a dynamic coefficient, providing a quantitative basis for assessing pedestrian safety in both dry and contaminated conditions as required by flooring specifications.
Specimen Preparation and Conditioning
Because friction is highly sensitive to surface contamination, specimens must be cut cleanly, handled by their edges, and kept free of fingerprints, dust, and creases. Films, paper, and board are conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity in accordance with ISO 291 or ASTM D618, typically for at least 40 hours before testing. Specimen dimensions must fully cover the sled base and provide sufficient plane length for the specified sliding travel, and separate specimen sets are prepared for each test pairing and direction.
Calculation and Data Evaluation
The kinetic coefficient is computed as μk = Fk / N, where Fk is the average force during steady sliding and N is the normal force, that is, the total weight of the sled assembly. For a standard 200 g sled, N is approximately 1.96 N, so a measured sliding force of 0.5 N corresponds to μk ≈ 0.26. The static coefficient is calculated analogously from the initial peak force. Results are reported as the mean and range of valid replicates, together with the test pairing, sled mass, sliding speed, sliding direction, and conditioning atmosphere, since all of these parameters materially influence the measured values.
Applications
- Packaging: optimizing film slip properties for vertical form-fill-seal and horizontal flow-wrap lines, ensuring reliable feeding, folding, and stacking at high production speeds.
- Additive and surface engineering: verifying slip-agent dosage and migration behavior, evaluating corona treatment, coatings, and embossed surface structures.
- Printing and converting: guaranteeing consistent paper and board feed in sheet-fed presses, copiers, and cartoning machinery.
- Flooring and construction safety: demonstrating adequate slip resistance of resilient, laminate, and coated floors for specification and regulatory acceptance.
- Automotive and medical products: controlling friction of seals, hoses, gloves, syringe components, and device handling surfaces.
- Quality control and incoming inspection: batch conformity testing of raw films, papers, and molded parts against agreed acceptance limits.
FAQ
What is the difference between the static and kinetic coefficient of friction?
The static coefficient describes the resistance to starting motion from rest and is obtained from the initial force peak, while the kinetic coefficient describes the resistance during continuous sliding and is obtained from the average steady-state sliding force. The static value is almost always equal to or higher than the kinetic value for the same material pair.
Which standards are most commonly used for dynamic friction testing?
ASTM D1894, ISO 8295, and GB/T 10006 are the leading methods for plastic films and sheeting, ASTM D4917 covers uncoated printing and writing papers, and EN 13893 addresses dynamic friction of floor coverings. The choice depends on the product type, the target market, and the referenced specification.
What coefficient of friction is suitable for packaging films?
For good machinability on packaging lines, a film-to-film kinetic coefficient of roughly 0.2 to 0.3 is commonly targeted. Values that are too low cause handling and stacking difficulties, while values much above about 0.4 increase the risk of film jamming and line stoppages. The acceptable range is always agreed between supplier and converter.
Why can the two sides of the same film give different results?
Slip and antiblock additives often migrate preferentially to one surface, and treatments such as corona, printing, or lamination alter one side only. In addition, the film side contacting the production roll differs in surface history. Both sides should therefore be tested and reported separately.
How do temperature and humidity affect the measurement?
Higher temperatures generally accelerate slip-agent migration to the surface and can soften polymers, which may lower or, in tacky materials, raise friction. Humidity influences paper and board significantly, since moisture softens fibers and increases surface adhesion. Strict conditioning and reporting of the test atmosphere are essential for comparable results.
Can the coefficient of friction be greater than 1?
Yes. The coefficient is a ratio of forces, not a percentage, so values above 1 are physically possible for tacky, adhesive, or highly hysteretic materials such as soft elastomers. Values measured under such conditions are strongly speed- and pressure-dependent, so test parameters must always accompany the reported number.